1. Field of the Invention
The present invention relates to the field of corrugating machinery and, more particularly, to a single facer for the bonding of a corrugated medium web to a liner web to form a single faced board.
2. Description of the Prior Art
A conventional single facer includes an upper corrugating roll and a lower corrugating roll wherein each roll includes a plurality of longitudinally extending teeth. The corrugating rolls are rotatably mounted so that the teeth are disposed in a meshing relationship. A medium web is supplied between the meshing teeth to corrugate flutes therein.
A starch-based glue is applied to the tops of the flutes by a gluing roll, arranged to turn in a bath of glue. Simultaneously, a liner web is fed from a side opposite the medium web over a pressure roll and brought into engagement with glued flutes of the corrugated medium web. A pressure roll as employed in a conventional single facer, is a large diameter metallic roll arranged adjacent the lower corrugating roll to apply a nip pressure to the corrugated web and liner web thereby effecting bonding therebetween.
Bonding of the corrugated medium and liner webs is a direct function of pressing duration and pressing force, wherein pressing duration is directly related to the length of the pressing nip and processing speed of the single facer. If the nip length is decreased while maintaining a constant processing speed then the pressing force must be increased to provide effective bonding. However, if the pressing force is decreased and the processing speed held constant then effective bonding necessitates that the nip length be increased.
Traditional pressure rolls provide a small nip length for acting against the corrugated web and liner web, thereby necessitating a high pressing force. Such high pressing force between the pressure roll and lower corrugating roll typically results in linear press marks, corresponding to the pitch of the teeth of the lower corrugating roll, being formed laterally on the surface of the liner web. The single faced boards produced by the prior art single facers therefore often have an unattractive appearance and are rejected as being defective.
In response, it has been proposed to replace the conventional pressure roll with an endless belt having a portion wrapped about the lower corrugating roll thereby forming an extended nip for pressing the liner web together with the corrugated medium web. More specifically, the endless belt is extended over a plurality of rolls to run freely in cooperation with the lower corrugating roll wherein the liner web and the corrugated medium web pass between the lower corrugating roll and the endless belt and are nipped therebetween.
A common problem associated with the prior art single facers employing an endless belt as described above, is that the belt cannot provide sufficient pressing force given the available nip length, due to machine structure constraints, to provide effective bonding of the liner web and the corrugated medium web. While the tension in the belt may be increased to thereby increase the pressing force, this tension is limited based upon the properties of the belt. Excessive tension in the belt may cause accelerated wear or tearing of the belt.
Accordingly, there remains a need for a single facer providing sufficient pressing force to effectively bond a liner web with a corrugated medium web without damaging the liner web. Further, there is a need for such a single facer providing for a variable auxiliary pressure nip independent of belt tension which may be adjusted in response to variations in web and operating conditions.